Fuel cells are electrochemical energy conversion devices considered as a possible alternative to internal combustion engines. Fuel cells convert a hydrogen containing fuel such as methanol or hydrogen to electrical energy by an oxidation reaction. A by-product of this reaction is water. Adequate output voltage entails the assembly of multiple fuel cells, connected in series, into fuel cell stacks.
Various proton exchange membrane (PEM) fuel cells have been described.
One type of PEM fuel cell comprises a solid polymer electrolyte (SPE) membrane, such as a sulfonated fluorinated polymer membrane material known as Nafion, which provides ion exchange between cathode and anode electrodes. Various configurations of SPE fuel cells as well as methods for their preparation have been described. See e.g. U.S. Pat. Nos. 4,469,579; 4,826,554; 5,211,984; 5,272,017; 5,316,871 5,399,184; 5,472,799; 5,474,857; and 5,702,755.
PEM fuel cells operate properly only if the membrane is sufficiently wet. A supply of water is required to retain proton conductivity of the polymer membrane and water management is essential for enhancement of cell performance (Choi et al. J. Power Source 2000 86:197-201; Fuller, T. F. and Newman, J. J. Electrochem. Soc. 1992 139:1332; Xie, G. and Okada, T. J. Electrochem. Soc. 1995 142:3057; Bernardi, D>M. J. Electrochem. Soc. 1990 137:3344; Sakai et al. J. Electrochem. Soc. 1986 133:88 and Yeo, R. S. and McBreen, J. J. Electrochem. Soc. 1979 126:1682).
Typically, hydration is maintained by humidification of the reactant streams prior to introduction into the electrochemically active regions of the fuel cell. Such systems also typically require heating of the fuel prior to its introduction into the fuel cell. A system providing both heat and humidification to the reactant stream prior to entry into the fuel cell is described in U.S. Pat. No. 4,530,886. The required humidification and heating apparatus of such systems, however, add complexity to the fuel system as components such as a humidification water pump, piping, a water reservoir and a filtration unit, in addition to the humidification module and heater, are also needed. Further, additional power is required for operating the pumps and heating the stream.
Thus, various attempts have been made to provide simpler, more energy efficient means for providing temperature control and/or humidification to fuel cells.
For example, U.S. Pat. No. 5,382,478 describes an electrochemical fuel stack with a humidification section located upstream from the electrochemically active section. The upstream location of the humidifier allows for fewer manifolds in the fuel cell, thereby maximizing space for the electrochemical reaction to occur.
U.S. Pat. No. 6,500,573 discloses a humidifier device for use with fuel cells comprising a mist humidifier unit for adding mists to process gas supplied to an electrolyte equipped in a fuel cell. This device is further equipped with a unit control device for intermittently operating the mist humidifier unit in accordance with operating condition of the fuel cell.
U.S. Pat. No. 5,958,613 discloses a polymer electrolyte fuel cell system with a polymer electrolyte fuel cell made up of a cell main body, a mixture generator for generating a gas-liquid mixture by mixing fuel gas which has been supplied from a fuel gas supply with water, and a means for supplying the gas-liquid mixture to the anode-side channels. The gas-liquid mixture permits moistening of the solid-polymer membrane without requiring a humidifier to humidify fuel gas and oxidant gas and cools the cell main body without providing a cooling channel therein.
U.S. Pat. No. 6,653,012 describes a simpler humidifier having a plurality of water-permeable hollow fiber membranes placed along the lengthwise direction of the housing of the humidifier. Gases, each having a different moisture content flow inside and outside the hollow fiber membranes to carry out moisture exchange so that dry air having low moisture content is humidified.
Humidification of fuel cells has also been provided using water transport plates. Water transport plates are porous structures filled with water. During fuel cell operation, the water transport plate supplies water locally to maintain humidification of a proton exchange membrane, removes product water formed at the cathode, removes by-product heat via a circulating coolant water stream, conducts electricity from cell to cell, provides a gas separator between adjacent cells and provides passage for conducting the reactants through the cell.
U.S. Pat. No. 4,973,530 discloses a fuel cell with a first flow field for transporting a fuel gas and a second flow field for receiving the gas which also has a water transport membrane for regulating humidity of the gas within the first flow field.
U.S. Pat. No. 5,965,288 also describes a gas-humidifying device for use with a fuel cell in which water permeable membranes are provided with fuel gas passages and oxygen-containing passages formed on one side of the water permeable membrane and humidifying water passages formed on the other side of the membrane. Medium supply passages are also provided independently from, yet in proximity to the water passages, for supplying a medium having a melting point lower than that of water so that melting of the water in the interior of the humidifier can be easily and swiftly carried out.
U.S. Pat. No. 6,197,442 discloses a water transport plate for use in improving fuel cell operations comprising graphite powder, reinforcing fibers, cellulose fibers and a thermosetting resin, the slurry of which is showered onto a screen to form a planar sheet which is dried to form paper. The paper is then cut into a desired size, laminated under heat and pressure, carbonized and graphitized to form a water transport plate.
U.S. Pat. No. 6,066,408 also describes a plate, referred to as a cooler-humidifier plate for use in a proton exchange membrane (PEM) fuel cell stack assembly. This cooler-humidifier plate combines functions of cooling and humidification with the fuel cell stack assembly, thereby providing a more compact structure, simpler manifolding, and reduced reject heat from the fuel cells. In this plate, coolant on the cooler side of the plate removes heat generated within the fuel cell assembly. On the humidifier side of the plate, evaporating water humidified reactant gas flows over a moistened wick. After exiting the humidifier side of the plate, the humidified reactant gas provides needed moisture to the proton exchange membranes used in the fuel stack assembly. Structural support is maximized in this plate by ensuring that the ribs that form the boundaries of channels on one side of the plate have ends at locations that substantially correspond to the location of ribs on the opposite side of the plate.
U.S. Pat. No. 6,632,555 discloses a proton electrolyte membrane fuel cell having a closed coolant path within the fuel cell, a humidifier with a humidification fluid flow path and a fuel and air gas supply passage continuous in, through and out the humidifier. In this system, the humidifier and the fuel and air gas supply passage are separated by a water permeable membrane impervious to organic materials, but which allows water from the humidification fluid flow path to enter the fuel and air gas supply passage.
Self-humidifying systems for fuel cells have also been described.
For example, U.S. Pat. No. 6,106,964 describes a solid polymer fuel cell system and a method for humidifying and adjusting the temperature of a reactant gas stream wherein heat generated by the fuel cell and water vapor in a reactant stream exhausted from the fuel cell are used to heat and humidify the reactant gas stream prior to introduction into the fuel cell.
U.S. Pat. No. 6,416,895 also describes a solid polymer fuel cell heated and humidified using heat generated by the fuel cell and water vapor from the fuel cell exhaust. This is achieved by flowing a reactant gas supply stream and a fuel cell exhaust gas stream on opposite sides of a water permeable membrane in a combined heat and humidity exchange apparatus.
The present invention provides modified fuel cells with internal humidification and temperature control systems.